Shenton, 24, confirmed he would be leaving The Jungle at the end of the season last month and Saints have wasted no time in securing his signature.
Saints chairman Eamonn McManus said: “Michael is one of the best young rugby league talents in England. Although already an established international, we feel that his best years very much lie ahead of him and that he will flourish at St. Helens. He has size and speed and will complement our back line very well.”
Shenton, who made his Super League debut in 2004 and scored 19 tries last season, said: “Although it’s a real wrench for me to leave Castleford, I feel that St Helens present a perfect opportunity for me to develop my rugby league career at the highest level.
“They have a long and great record of success and play a brand of rugby that will suit me. It’s a very exciting time to be joining the club and I’m really looking forward to the challenge.”